The present invention relates to a thin plate made of an Fe--Ni alloy for electronic parts, which can have lower yield strength and lower hardness by annealing heat treatment under conditions of a lower temperature and a shorter time, a shadow mask made from the thin plate, and a cathode-ray tube provided with the shadow mask. Herein, the term softening treatment"means an annealing treatment performed for softening purpose.
The Fe--Ni alloy has excellent thermal expansion property, thus it has been utilized for the shadow mask of the cathode-ray tube for various display devices.
In the case where the shadow mask is made from the Fe--Ni alloy material, for example, a thin Fe--Ni alloy plate having a thickness of not more than 0.3 mm is prepared and subjected to an etching treatment in order to form minute holes through which electron beam passes. Thereafter the etched thin plate is subjected to bending working to provide it a form of the shadow mask. The obtained shadow mask is incorporated in a cathode-ray tube. In order for realizing the cathode-ray tube which can produce a high resolution image, the shadow mask is required to have a shielding effect with high accuracy against electron beam. Therefore the shadow mask material is required to have a good processability with very high accuracy in etching treatment and press forming. In order to obtain such properties of the shadow mask material, there have been proposed various ways such as reducing inclusions, adjusting of crystal grain size and controlling of crystal orientation.
As one of the ways for improving the accuracy in press forming, there was proposed a warm press forming method by JP-B2-5-49727, in which a material being provided with through holes for electron beam by etching is subjected to forming as heated. It is effective to form the material within a warm temperature range for reducing the yield strength and improving the forming accuracy.
Further, there has been a practice of subjecting materials to softening treatment at not lower than about 800.degree. C. previous to the press forming mentioned above or cold rolling. Such softening treatment is effective in fully softening materials to readily carrying out the press working.
But, since it is not preferable economically to perform the softening treatment at a high temperature higher than about 800.degree. C. for a long time, there has been a demand for developing a material which can be softened at a softening temperature as lower as possible in a time as shorter as possible.
JP-A-7-48651 teaches a material which is subjected to such softening treatment and which is improved in the processability of press forming by reducing the amount of oxygen to decrease oxide inclusions. It is effective to reduce oxide inclusions for improving the processability of press forming. However, such processability can not be enough improved by reducing non-metallic inclusions such as oxides. It is also economically disadvantageous to produce a material being reduced in non-metallic inclusions taking account of a huge cost with respect to manufacturing conditions in the refining process and so on.